1. Field of the Invention
The present invention pertains to supported olefin polymerization catalysts employing a single site precatalyst, an ionic activator, and a metal alkyl alkylating agent, and olefin polymerization processes employing such catalysts.
2. Background Art
Olefin polymerization for many years involved the use of so-called Ziegler-Natta transition metal catalysts and related transition metal complexes. Such catalysts generally required an xe2x80x9cactivatorxe2x80x9d such as a trialkylaluminum compound for olefin polymerization. Large quantities of polyethylene are still prepared using such processes.
In the decade of the 1970""s, it was discovered that organometallic compounds such as bis(cyclopentadicnyl) complexes of titanium, zirconium, and hafnium are also olefin polymerization catalysts, but their polymerization activity was very low with traditional activators. Instead, a variety of organoalumoxanes, particularly methylalumoxane, were discovered to be efficient activators. These catalysts have been termed xe2x80x9csingle site catalysts,xe2x80x9d as polymer growth is believed to be initiated and promulgated from individual catalytic sites, each site corresponding to a well structured xe2x80x9cprecatalystxe2x80x9d derived site. The term xe2x80x9cprecatalystxe2x80x9d may be used to emphasize that the metal complex itself is not usually the active catalytic species. Rather, the active catalyst is obtained following reaction with one or more of the additional active ingredients of the catalyst system, i.e. alumoxane, trialkylaluminum, and/or source of bulky, non-coordinating anionic ligand as described hereafter.
In U.S. Pat. Nos. 5,408,017; 5,198,401; 5,599,761; 5,470,927; 5,384,299; and 5,064,802, alternative catalyst systems are disclosed which employ single site catalysts activated by bulky, non-coordinating anions such as the tetrakis(pentafluorophenyl)borate anion and anions derived from Brxc3x6nsted acids. These catalyst systems, unlike those activated by alumoxane, generally employed trialkylaluminum as a xe2x80x9ccoactivator,xe2x80x9d xe2x80x9ccocatalyst,xe2x80x9d or xe2x80x9cscavenger,xe2x80x9d added as a feed stream to the polymerization reactor. Uncertainty in the actual function of trialkylaluminum compounds and related compounds is reflected in the variety of terms used to describe them. These terms are viewed as synonyms herein unless indicated otherwise.
Considerable research has been devoted to single site catalysts which have been activated with alumoxanes as xe2x80x9ccocatalystsxe2x80x9d or xe2x80x9cactivators.xe2x80x9d In solution polymerization, the alumoxanes may be added separately to the polymerization reactor; may be mixed with the xe2x80x9cprecatalystxe2x80x9d in large excess to form a catalyst solution, generally in organic solvent; or may be prereacted with limited quantities of alumoxane and isolated as a solid catalyst prior to introduction into the reactor.
In slurry and gas phase polymerization, however, supported catalysts are used. Supported catalysts are prepared by treating a finely divided, porous inorganic support, preferably silica, with the precatalyst or with both precatalyst and activator. Thus numerous possibilities exist for the preparation of alumoxane-activated supported catalysts, including, inter alia, deposition of only precatalyst on the support, the alumoxane being added directly to the polymerization reactor, and deposition of both precatalyst and alumoxane on the support.
In U.S. Pat. No. 5,332,706, it is disclosed that catalyst activity is enhanced if alumoxane and precatalyst are first contacted in solution, and this solution is added to porous silica in a quantity such that the solution volume relative to the pore volume of silica prevents obtaining a paste or slurry. This process may be termed an xe2x80x9cincipient wetnessxe2x80x9d process. The incipient wetness process has not been uniformly used, even with alumoxane cocatalysts.
In the case of bulky anion-activated precatalysts, the trialkylaluminum xe2x80x9ccocatalystxe2x80x9d or xe2x80x9cactivatorxe2x80x9d has been added to the polymerization reactor as a separate stream to alkylate the precatalyst in situ, or in a feed stream with a large volume of alkylating agent solution, for example as disclosed in copending U.S. application Ser. No. 6,291,386, herein incorporated by reference. Although polymerization activity increased, polymer bulk density, an important parameter, was not optimal, in some cases showing a considerable decrease in bulk density.
It would be desirable to provide a supported catalyst which maintains or increases polymerization activity relative to precatalyst content, which maintains its activity during storage, which can provide polymer bulk density which is improved over other methods of catalyst preparation, and/or which can provide polymer products with lower polydispersity.
It has now been surprisingly and unexpectedly discovered that a catalytic system comprising a metal complex single site precatalyst and a bulky, non-coordinating anionic ligand, when alkylated in the presence of a support by an incipient wetness technique, produces a supported catalyst whose activity is higher than the catalytic activity obtained when alkylating agent is added separately to the polymerization reactor, and can generate polymer with both lower polydispersity as well as improved polymer particle morphology as manifested by polymer bulk density.